Delamination of single layer hollow fiber membranes induced by bi-directional phase separation

Structural delamination across the hollow fiber wall is a major challenge for the fabrication of dual-layer hollow fiber membranes. Delamination may also exist in single-layer hollow fiber membranes because the dope solution undergoes phase separation at both inner and outer surfaces. Different phase inversion mechanisms and rates at the inner and outer surfaces result in hollow fibers with different morphologies across the wall and may lead to delamination between the layers. In this work, the origins of delamination for single-layer hollow fiber membranes are investigated. The effects of dope composition, bore fluid chemistry as well as spinning conditions on delamination are disclosed. Among these parameters, bore fluid composition has the most profound impact on fiber delamination. With carefully designed spinning conditions (e.g. high air gap and take-up speed), the occurrence of fiber delamination can be mitigated. A membrane with a delaminated inner layer often possesses a highly porous inner surface with an improved permeance for water filtration. This work offers feasible solutions to resolve structural delamination of hollow fiber membranes during spinning. It may also provide a new method to fabricate high-flux hollow fiber membranes by producing fiber delamination intentionally.